1. Field of the Invention
The present invention relates to a multi-link administrative telephone and intercom system having automatic as well as supervised call distribution and PBX capability.
2. Description of the Background Art
Dahlquist et al. U.S. Pat. No. 3,809,824 discloses a multi-link private automatic telephone system including "administrative" dial telephones and "staff" dialless telephones. The lifting of a receiver of a dialless telephone produces a visual indication on an annunciator panel. An administrator must respond by dialing the phone number of the dialless telephone in order to establish a communication link. The administrator may also dial other phone numbers to add other telephones to the link to establish a conference call or to permit a conversation between two dialless telephones.
Dahlquist et al. U.S. Pat. No. 4,081,614 discloses a single link telephone system including an "administrative" tone dialing telephone, "staff" dialless telephones, and intercom speakers. The administrative telephone includes a digital display for sequentially indicating the numbers of call-ins from the staff telephones or intercom speakers. To call the first number on the display, the administrator can merely press a single button on the administrative telephone. When a staff telephone or intercom speaker is called, its number is removed from the digital display. Each staff telephone or intercom speaker can transmit a priority call-in signal which places its phone number in the first display position and activates a visual and audible signal to attract the administrator's attention.
Microcomputer control is now being used for multi-link automatic private or private branch exchange (PBX) telephone systems. The microcomputer is used for assigning links to the system, and for diagnostic and reporting functions. A universal problem encountered when employing a microcomputer in an automatic telephone exchange is the interconnection of the microcomputer to the voice switching positions or circuits which connect the telephones to selected audio links. In addition busy signals, ringing signals, and "off-hook" signals must be conveyed between the microcomputer and the telephones. Also, it is desirable to provide flexibility to vary the size of the system and to modify the functions of the different stations. Typically these capabilities have been provided by complex or relatively expensive interface circuity.
One way of dealing with the microcomputer interface problem is to employ a number of microprocessors which communicate with each other on an asynchronous basis and which are interfaced to an assigned group of stations, as disclosed in Pitroda et al. U.S. Pat. No. 4,289,934. Another known method is to transmit only digital information between the phones as well as the microcomputer, and to provide each phone with audio-to-digital and digital-to-audio converters. This latter technique provides the greatest flexibility, but at a corresponding expense.